Ignition device for oil-fired boilers



Feb. 11, 1969 B. ANDRESS ETAL 3,427,118

IGNITION DEVICE FOR OIL-FIRED BOILERS Filed July 1, 1966 Sheet Fig.1

Feb. 11, 1969 AN-DRESS ET L IGNITION DEVICE FOR OIL-FIRED BOILERS Sheet Filed July 1. 1966 United States Patent 3,427,118 IGNITION DEVICE FOR OIL-FIRED BOILERS Bernhard Andress and Ludwig Kuchelbacher, Erlangen, Germany, assignors to Siemens Aktiengesellschaft, Erlangen, Germany Filed July 1, 1966, Ser. No. 562,233 Claims priority, application Germany, July 2, 1965,

Us. Cl. 431-258 3 Claims Int. c1. F23q 7/06; F02c 7/26 ABSTRACT OF THE DISCLOSURE Our invention relates to a device for igniting the oil mist of a boiler operating with crude oil in which a sufficiently large volume of an ignitable oil-air mixture is heated up to its ignition temperature.

Known devices of this kind place a powder charge into the combustion chamber and electrically ignite the charge to thereby ignite the oil-air mixture. Thereafter the carrier of the powder charge must be moved out of the combustion chamber.

It is an object of our invention to provide an ignition device operation that affords maintaining the ignition device completely separated from the combustion chamber not only before and after the ignition but also during the ignition interval.

Another, conjoint object of the invention is to provide an ignition device for oil-heated boilers that minimizes the wear imposed upon the device thus reducing maintenance requirements and prolonging its useful life.

According to the invention, we provide an oil-air ignition device which effects ignition by strongly bunched radiation of an optical transmitter released by a source of excitation and operating with a selectively fluorescent medium, such as embodied by radiation from a laser.

The invention is predicated upon the recognition that, with a sufficient energy density in a sutficiently large volume of oil mist, the amount of laser light absorbed by the mist will suffice to ignite the oil-mist volume, and that the thus initiated combustion will continue after cessation of the ignition pulse.

Before describing an embodiment of an iginition device according to the invention, it will be helpful to consider its principles. To afford igniting furnace oil by laser pulses, essentially three requirements must be met:

('1) The oil must sutficiently absorb the laser radiation,

(2) The laser radiation must impinge upon an ignitable mixture of oil and air,

(3) The energy density of the laser radiation must be sufficiently large to attain the ignition temperature of the oil, and this energy density must be attained by shortlasting ignition pulses within a sufiiciently large volume of the oil-air mist, so that after decay of the ignition pulses the generation of heat within this volume is larger than the radiation losses, thus causing the combustion to continue.

Patented Feb. 11, 1969 The invention will be further described with reference to the accompanying drawings in which:

FIG. 1 is an explanatory graph;

FIG. 2 shows schematicallyand in section an ignition device according to the invention; and

FIG. 3 is a schematic circuit diagram of the same device.

To afford judging the amount of energy transmission fromthe laser radiation to the furnace oil, the absorption of an oil in liquid form was determined in the spectral range of 0.5- 2 ,um. The result is illustrated in the diagram shown in FIG. 1 in which the abscissa indicates the wave length (A) of the laser light in micron (,um.) and the ordinate indicates the median penetration distance w in am. of the laser light, which is identical with the reciprocal value of the absorption constant K of the furnace oil, the ordinate value being represented on a logarithmic scale. It follows from FIG. 1 that it is favorable to employ a laser radiation of shortest feasible wave length. The radiation of a ruby laser is therefore better suitable than that of a neodymium-glass laser. For the wave length of the ruby, the median penetrating depth w in the oil film amounts to 36 m. corresponding to an absorption constant of K=2 8 mm.- An ignitable oil-air mixture is present if the oil is heated over C. (flame point 80- C.) and the volumetric ratio of oil (gaseous) to air is a few percent. Relating to the liquid oil, this corresponds approximately to an oilzratio of 1:1000. When furnace oil is atomized to a mist with the aid of a spray nozzle, similar dilutions are obtained. This is tantamount to the fact that with this dilution the laser radiation is absorbed up to 37% upon passing through a path length of about 36 mm. Consequently the median penetrating depth w of the laser light in the oil mist is approximately 36 mm.

Due to the relatively slight absorption of the laser radiation in the oil-air mist an essential proportion of the radiation is absorbed only upon passage through a layer thickness of a few mm. When employing short light pulses, for example, of 0.3 to 2 ms. duration, it cannot be expected that (luring the ignition pulse, having a propagation speed of about 1000 mrn./s., the ignited volume will increase appreciably. To ensure reliable spreading of the ignition over the entire oil-air volume after decay of the ignition pulse, which may require a period of a few one-hundredths of one second, the energy density required for ignition (i.e., heating of the oil droplets to 600-800 C. or higher temperatures) must be attained within a sufliciently large volume. This volume may be smaller with a higher ignition velocity in the particular fuel-air mixture being used, or with an increase in the density of the oil mist, or with an increase of the laserpulse duration or the energy density.

In FIG. 2 there is shown a boiler wall 1 of a boiler operating with crude oil. The wall has an opening 2 for the passage of an oil burner 3, and an opening 4 traversed by the outlet tube 5 of a laser device. The head portion 6 of the laser device is equipped with a lens 7 and a displaceable diaphragm 8. The laser head further contains a ruby laser crystal 9 and a flash lamp v1t) to serve as source of excitation energy. The ignition device for the flash lamp comprises a capacitor 11 and an ignition coil, further a power supply unit 12 (FIGS. 2, 3) which is to be connected to a utility line and contains a capacitor battery 15 for energizing the flash lamp 10. The focal length of the lens 7 is 1 meter.

The tube 5 and the lens 7 are arranged to focus the laser radiation into the oil mist 13. The ru-by crystal has an opening angle of about 30'. Consequently, the laser beam diameter in the region of highest energy density is approximately 2 mm. The region of highest energy density, resulting from the coaction of parallel and divergent radiation, extends over a length of a few cm. This length is in accordance with approximately the median penetrating depth of the laser light in the oil-air mist.

Referring now to FIG. 3, the power supply unit 12 is shown to comprise a transformer 14 which serves to step up the line voltage to approximately 1.5 -kv. Connected to the secondary winding of the transformer are the capacitor battery 15 and a rectifier 16. The capacitor battery '15 becomes charged nearly to the peak value of the secondary alternating voltage. The ignition unit 11 for the flash. lamp 10 comprises an ignition coil 17 which receives a voltage pulse in the order of magnitude of 10 kv. from a circuit 18 containing a spark gap 19. This voltage pulse ignites the flash lamp 10'. The energy for the flash lamp is then furnished from thecapacitor battery 15. The switch for the ignition pulse is denoted by 20. When it is being closed, it also closes the circuit of a lifting magnet 21 which raises the displaceable diaphragm 8 and thus exposes the lens 7 (FIG. 2) to permit the laser beam to pass into the combustion chamber. A normally open contact 22 on the armature of the lifting magnet 21 closes arsenide Laserdiodes by Henkel et al., in Solid State Elec-v tronics, Pergamon-Pres s, 1965, vol. 8, page 475 and by Gremmelmaier and Henkel in Siemens Zeitschrift, vol. 39, No. 5, 1965 palge 438,. as well as to the bibliographies in thelatter two papers.

To those skilled in the artitwill be obvious from a study of this disclosure that our invention permits of various modifications andmay be given embodiments otherthan' particularly illustrated and described herein,

the circuit for the input transformer 23 of the ignition device. This affords the assurance tha t the ignition pulse will be released only after the diaphragm 8 has reached the open position. Consequently, when the switch, 20 is- The described ignition device affords igniting the furnace oil, heated to 80 Crand finely dispersed into droplets, by applying laser pulses of approximately 1.5 Ws energy and a 0.5 ms. duration. Similar results were obtained when reducing the focal length of the lens down to 10 cm. The lens 7 also serves as a protective window be- 1 tween the laser crystal and the combustion chamber of the boiler. For this purpose it is preferably protected by the displacea'ble diaphragm 8 from being soiled. The lens becomes exposed only during the ignition interval. For that reason, the diaphragm 8 or rather the actuator for displacing it, is preferably connected with the ignition switch of the laser or is displaced by an electromagnetic con trolled by the ignition switch, as exemplified by the circuit shown in FIG. 3.

The laser device does not require any particular cooling of the crystal and the flash lamp becauseit is operated only'with individual pulses of relatively large time spacing from each other. Ruby crystals of good quality issue the required median pulse energies at 20 C. and thus permit operating at normal room temperature.

The device and its proper operation are not limited to the use of ruby or neodymium-glass lasers. Other solidstate lasers or semiconductor injection lasers arealso applicable, provided they satisfy the above-described requirements. The emission may also be outside of the visible spectral region.

Such and other types of lasers applicable for the purposes of the invention are known as such, for example without departing from the essential features of our invention and within the scope of the claims annexed hereto.

1. In combination with a boiler fired by crude oil, an ignition'device comprising a source of laser radiation and excitation means for controlling said source to issue pulses of optical lradiation, optical means near said source for concentrating a beamof' said radiation pulses onto an oil-air mixture to be ignited'so as to produce an energy density sufficiently large to attain the ignition temperature of the oil in a volume of the oil-air mixture sufficiently large so that, after decay of said radiation pulses, generation of heat within said volume is gerater than heat loss therefrom whereby combustion of the oil-air mixture is maintained, and holder means on which said source and .said optical means are mounted, said holder means forming a-beam outlet and being adapted for attachment to a firing chamber of the boiler in igniting relation to the oil supply.

2. The combination according to claim 1, said holder means comprising a tube having an opening at one enddefining said beam outlet, and said optical means comprising a focussing lens coaxially mounted in said tube be tween said source and said opening.

3. The combination according to claim 2, said lens having a focal length of about 10 to 100 cm.

References Cited UNITED STATES PATENTS 1,625,630 4/1927 Scott. 2,332,210 10/1943 Frank. 2,602,293 7/1952 Goddard. 3,177,651 4/1965 Lawrence. 3,296,795 1/1967 Nielsen.

OTHER REFERENCES Rockets, Oct. 1945, page 10.

'IAM-ES W. WESTHAVER, Primary Examiner.

U.s. c1. X.R. 6039.82 

